The invention relates to a manufacturing process for hollow pressure vessels, particularly compressed gas cylinders made of aluminum alloy AlZnMgCu, in other words in the 7000 series according to the Aluminum Association""s nomenclature.
The use of aluminum alloys in the 7000 series for manufacturing hollow pressure vessels has been known for many years; these alloys have a high mechanical strength in the heat treated temper, such that the weight of the manufactured product can be reduced. Manufacturing includes casting and homogenization of billets, reverse extrusion of a cylindrical casing, tapering of the cylinder neck and heat treating by solution heat treatment, quenching and aging. The other properties required for this application are formability, particularly for the cylinder necking operation, good resistance to stress corrosion and inter-crystalline corrosion, and obtaining a ductile behavior during bursting tests under internal hydraulic pressure.
Patent FR 2510231 filed by the applicant describes the use of a 7475 type alloy for this application with the following composition (% by weight):
Zn=5.6-6.1, Mg=2.0-2.4, Cu=1.3-1.7, Cr=0.15-0.25, Fe less than 0.10, Fe+Si less than 0.25. The reverse extrusion operation may be done hot or cold.
Patent EP 0081441 filed by the applicant describes a process for manufacturing high strength and high toughness extruded products made of 7049A alloy with the following composition:
Zn=7.2-9.5, Mg 2.1-3.5, Cu=1.0-2.0, Cr=0.07-0.17, Mn=0.15-0.25, Fe less than 0.10, Si less than 0.08, Zr=0.08-0.14.
The product is extruded at a temperature of the order of 400xc2x0 C.
Patent EP 0257167 filed by the applicant describes the use of a 7060 alloy with the following composition:
Zn=6.25-8.0, Mg=1.2-2.2, Cu=1.7-2.8, Cr=0.15-0.28, Fe less than 0.20, Fe+Si less than 0.40, Mn less than 0.20.
Patent EP 0589807 is a variant of the previous patent in which Cr is replaced by Zr (0.10-0.25%). Cylinders made of 7060 are produced industrially by hot extrusion.
Patent application WO 94/24326 by Alcan International describes a process for manufacturing a hollow pressure vessel starting from an alloy with composition Zn=5.0-7.0, Mg=1.5-3.0, Cu=1.0-2.7, Fe less than 0.30, Si less than 0.15, a recrystallization inhibitor (particularly Cr or Zr) with a content 0.05-0.4, with a microstructure such that the fraction of the S phase (CuMgAl2) by volume is kept below 1%, and preferably below 0.2%. According to the application, this microstructure is obtained by homogenization of the billet at about 475xc2x0 C. with a low rate of temperature rise when getting close to this value. Preferably, for cost reasons, extrusion is done cold or slightly warm. Here, aging is an overaging which is applied such that the yield strength is about 20% below the peak to improve the toughness, fatigue resistance, and resistance to crack propagation and stress corrosion. An alloy with the claimed composition was subsequently recorded at the Aluminum Association with the designation 7032.
Patent application EP 0670377 made by Pechiney Recherche applies to alloys with a high mechanical strength and the following composition:
Zn=7-13.5, Mg=1.0-3.8, Cu=0.6-2.7, Mn less than 0.5, Cr less than 0.4, Zr less than 0.2
possibly transformed by extrusion to obtain hollow vessels. Homogenization and solution heat treating are carried out at 10xc2x0 C. below and preferably 5xc2x0 C. below the incipient melting temperature under conditions such that in T6 temper, the absolute value of the specific energy associated with the DTA (differential thermal analysis) signal is less than 3 J/g.
For some applications, it is desirable to use very high strength alloys to give the minimum weight of cylinders, and also to reduce manufacturing costs; for example this is the case for portable extinguishers. One means of lowering the cost is to use cold extrusion, in other words in which the metal at ambient temperature at the beginning of extrusion, or slightly warm extrusion in which the metal is heated before extrusion to a temperature of less than 300xc2x0 C., which is significantly more economic than hot extrusion in which the metal is heated to between 350 and 450xc2x0 C. before extrusion.
However, cold extrusion of high strength alloys such as 7060 causes very high extrusion forces that are often incompatible with extrusion presses usually used for this type of product, or in any case reducing the life of extrusion tools. Furthermore, application of the information in WO 94/24326 to the 7060 alloy concerning the billet homogenization temperature (more than 470xc2x0 C.) often means that the alloy incipient melting temperature is reached during homogenization.
Thus, the purpose of the invention is to develop a procedure for manufacturing high strength hollow pressure vessels made of a 7000 alloy, such as 7060 alloy, by cold or slightly warm extrusion under acceptable industrial conditions, in order to give a high mechanical strength without prejudice to other properties required for this application.
The object of the invention is a process for manufacturing hollow pressure vessels, particularly compressed gas cylinders, comprising the following steps:
a) cast a billet using an alloy with the following composition (% by weight) Zn=6.25-8.0, Mg=1.2-2.2, Cu=1.7-2.8, Fe less than 0.20, Fe+Si less than 0.40, Mn less than 0.10, at least one of the elements belonging to the group consisting of Cr, Zr, V, Hf, Sc in the proportion 0.05-0.3, other elements  less than 0.05 each and  less than 0.15 total,
b) homogenization of this billet using a temperature profile such that the metal temperature is slightly less than its incipient melting temperature at all times,
c) softening annealing with a duration of 20 to 40 h between 200 and 400xc2x0 C. with cooling at less than 50xc2x0 C./h down to a temperature of below 100xc2x0 C., such that the hardness  less than 54 HB,
d) cutout a slug,
e) cold or slightly warm extrusion (extrusion start temperature  less than 300xc2x0 C.) of a casing,
f) necking the casing,
g) solution heat treting at a temperature slightly below the incipient melting temperature, with a duration such that the absolute value of the specific energy associated with the DTA signal is less than 3 J/g (and preferably  less than 2 J/g)
h) quenching in cold water,
i) aging at between 100 and 200xc2x0 C. for a duration between 5 and 25 h.
The chemical composition of the alloy is within the limits defined in patents EP 0257167 (chromium alloy) and EP 0589807 (zirconium alloy). Chromium and zirconium may be replaced by vanadium, hafnium or scandium. Preferably, the contents will be (individually or in combination) Zn greater than 6.75%, Mg less than 1.95%, Fe less than 0.12%, Fe+Si less than 0.25%, Mn less than 0.10%.
The alloy is cast in billets in a manner known per se, for example by semi-continuous casting.
Homogenization is done with a temperature profile such that the alloy temperature is a few degrees C below the incipient melting temperature of the alloy, that may vary from 470 to 485xc2x0 C. depending on the alloy composition, at all times. It is important that homogenization is sufficient, otherwise there is a risk of seeing cracks appearing during extrusion due to the alignment of coarse copper phases (for example AlCuZn) and causing dissolution of local melting, leading to decohesions, burning or porosity. The homogenization quality may be evaluated by differential enthalpic analysis. Insufficient homogenization will cause initial melting with a large endothermic peak, indicating metastable eutectic melting (xcex1Al+S, M, T). It can be estimated that this quality is good when, as described in patent EP 0670377, the DTA thermogram indicates an absolute value of the specific energy associated with the melting peak equal to less than 3 J/g, and preferably less than 2 J/g. It will also be possible to make this check on the solution heat treated product only, and then judge the quality of the homogenizationxe2x80x94solution treatment pair.
It is important that the incipient melting temperature should not be reached if good ductility is to be obtained. Preferably, this is done by homogenization in two isothermal steps at increasing temperatures. The temperature of the first step also depends on the alloy composition. It is estimated that if the composition is such that %Mg less than 0.5%Cu+0.15Zn, the temperature of the first step must not exceed 465xc2x0 C., and when Mg greater than 0.5Cu+0.15Zn, it must not exceed 470xc2x0 C.
The hardness of billets homogenized in this manner is high and very large forces on the press are necessary during cold or slightly warm extrusion, which reduces the life of tools. This is why it is essential to perform softening annealing that produces an acceptable hardness level, that may be equal to 54 HB, this Brinell hardness being measured with a 2.5 mm diameter ball and a 62.5 kg load. This annealing preferably includes several isothermal steps at decreasing temperatures between 400 and 200xc2x0 C. with a total duration of between 20 and 40 h followed by a fairly slow temperature drop, less than 50xc2x0 C./h, down to a temperature  less than 100xc2x0 C. The hardness obtained on softened billets no longer changes by maturation at ambient temperature.
The softened billets are then cut into slugs corresponding to the quantity of metal necessary to obtain a cylinder blank in the form of a cylindrical casing by cold or slightly warm extrusion. A tapering operation is performed that consists of forming the cylinder neck by necking.
The part obtained is then solution heat treated at a temperature as close as possible to the incipient melting temperature of the alloy, while avoiding burning. The solution heat treatment quality, that depends both on the quality of prior homogenization and the solution treatment conditions themselves, is also evaluated by differential enthalpic analysis on samples in the T6 temper. The specific energy (absolute value) associated with the melting peak of the DTA thermogram must be less than 3 J/g and preferably  less than 2 J/g, regardless of the location from which the sample is taken on the cylinder. The result may be different for the top and bottom of the cylinder due to the difference in the cooling rate during quenching. If the cylinder is dipped into the quenching liquid with the top part first, then the top will be cooled quickly whereas the bottom will be cooled more slowly.
Aging is done at a temperature of between 100 and 180xc2x0 C. for a duration of between 5 and 25 h. This aging preferably consists of two isothermal steps at increasing temperatures, the first at a temperature of between 100 and 120xc2x0 C. for 4 to 8 h, and the second at a temperature of between 150 and 180xc2x0 C. for between 5 and 20 h. This aging must be done to give a good compromise between the mechanical strength that decreases when aging is done for a longer period, and resistance to corrosion and particularly stress corrosion, that increases with averaging. After aging, the result is a recrystallized fine grain structure that gives excellent ductility.
The process according to the invention can give a remarkable set of properties, namely ultimate tensile strength Rm greater than 490 MPa, guaranteed yield strength R0.2 greater than 460 MPa, elongation A greater than 12%, lack of inter-crystalline corrosion, no break at 30 days due to stress corrosion at 350 MPa, while using a cold or slightly warm extrusion technique that is more economic than hot extrusion under acceptable industrial conditions.
The process is applicable to the manufacture of high pressure cylinders designed particularly for extinguishers, gas for breweries, breathing apparatus, industrial gases. It is economically adapted to the production of cylinders for single use only, which simplifies distribution. It is also applicable to the manufacture of metallic liners for composite wound cylinders using glass, carbon or aramid fibers.